I have a 45,000 gal. inground plaster pool. I have a light brown stain that is starting to form on the bottom and sides. I covered the pool for the first time this year and upon opening. It was clear and clean. I was zero on Chlorine upon removing the cover everything else was in balance. I filled (about 8 inches of city water), shocked and started the pump and chlorinator. Never lost clarity, in the process. But, the light brown lines started to appear, particualry in the shallow end. Allmost looked like a natural wave line as it's form. I have tried to brush, (no change) I have added algaecide, (just in case) and run a very high chlorine level. I took a lemon and washed a spot and it did nothing. Went to the pool store and got a package of citris spot checker and nothing. Any suggestions. I've had no problem for the past year with my system. I do have older Copper pipes feeding the pool. thanks.. Jerry